Nucleic acid hybridization studies using cloned retroviruses, DNA's and recombinant DNA techniques are being used to examine the sites of integration and organization of viral sequences in cells and tissues. The principal goal of these studies is to develop probes that will allow the detection of RNA tumor viral sequences in normal and malignant human tissue. These same techniques are being employed to examine the organization of the two endogenous feline viruses (RD and FeLV) in various backcrossed animals which are offsprings of a genetic cross between an F1 hybrid and a virogene-negative parent (leopard cat).